In evaporable foam casting methods, a pattern formed of an evaporable foam material, such as polystyrene, is positioned within an outer mold and a flowable material, such as sand, is introduced into the mold to surround the pattern and fill the internal passages in the pattern. When a molten metal is introduced into the mold, the molten metal will vaporize the foam pattern, with the vapor passing into the interstices of the sand, while the molten metal fills the void produced by the evaporation of the pattern. The resulting cast metal article has a shape identical to the foam pattern.
In normal practice, the pattern, as well as the sprue and the gating connecting the sprue to the pattern, is coated with a refractory material which normally consists of a suspension or slurry of aluminum silicate in water and containing a binder, such as clay. The coating, after drying on the pattern, governs the fill rate by controlling the flow of vapor into the sand during the casting operation.
In conventional processes, the coating is applied to the pattern by dipping, brushing, or flow coating, and the wet coated patterns are suspended by hangers from racks or from a conveyor. The racks or conveyors are then passed through a gas fired or electrically heated oven to dry the coating material. The oven must be maintained at a temperature sufficiently low to prevent any adverse effect on the foam pattern material, the coating, and any adhesives that may be present.
The conventional drying method allows for wide variations in solvent removal rates. Internal passages and recesses are isolated from air currents and tend to saturate the air in the passages, thus retarding evaporation of the solvent or carrier. This effect is particularly significant in water based slurry coatings. Because most pattern systems must be maintained at temperatures below 160.degree. C., it has been found that external surfaces of the pattern may dry at approximately three times the rate of drying of internal isolated passages.
In an attempt to equalize the drying rate on external and internal surfaces it has been proposed to utilize convection fans in association with ovens to direct air to internal passages in the patterns. However, the specialized fixturing which is necessary to utilize convection fans is not feasible from an economic or operational standpoint.
It has also been proposed to dry water based coatings through the use of microwave energy and to concentrate the energy on those areas of the pattern, such as internal passages, which are difficult to dry. However, the use of microwave energy has not proven satisfactory, in that is is difficult to control the energy transmission to avoid overheating and thus damaging the pattern or other item being dried. Secondly, the use of microwave energy does not address the situation that the internal passages may be saturated with the solvent or carrier, with no method of removing the saturated vapor. A further concern is the cost and safety issues associated with utilizing microwaves in an industrial environment.